The present invention relates generally to magnetic resonance imaging (MRI) devices, and more particularly to MRI devices including at least one gradient coil for manipulating the magnetic field generated by the MRI magnet, wherein the magnetic fields generated by the gradient coil are substantially magnetically unshielded.
MRI devices are widely used in the medical community as a diagnostic tool for imaging items such as tissue and bone structures. Conventional MRI devices are described, for example, in U.S. Pat. Nos. 5,225,782; 5,285,181; and 5,304,934 which are all incorporated by reference herein in their entirety.
As shown in FIG. 1, known superconducting (SC) MRI devices 10 typically employ windings 30 for generating a homogeneous magnetic field in an image volume 20, the windings 30 operating in liquid helium to maintain the temperature at approximately 4° K. The liquid helium pool requires a vessel 40 which is vacuum tight and which meets American Society of Mechanical Engineering (ASME) pressure vessel requirements; such a vessel 40 is typically made of welded aluminum alloy cylinders and flanges. Thermal radiation shields (not shown), of which two are typically used, are also made of welded aluminum pieces and contain the helium vessel 40.
When the gradient coils 50 in the bore of the MRI device 10 are electrically pulsed, the resulting time changing magnetic flux in any of the electrically conducting cylinders surrounding the gradient coils induces eddy currents. These eddy currents in turn produce their own magnetic fields which degrade the quality of the desired gradient field in space and time. A second set of gradient coils 60 (i.e., shield gradient coils) in the magnet bore compensate for the aggressive pulse sequences which are routinely used in MR imaging today. These shield gradient coils 60 set up fields which counteract those of the main gradient coil 50 in the region outside of the shield coil 60, thus greatly reducing any mutual inductance with conducting members, such as the thermal shields, and minimizing the resultant eddy currents. The present inventors have found that, in a typical implementation, the shield coils 60 generally cancel about 50% of the magnetic field produced by the gradient coils 50.
A need exists, however, for a MRI device 10 which reduces the amount of resultant eddy currents produced in the MRI device 10 by the gradient coils 50 in systems without the shield coils 60, or, for systems with shield coils, further reduces the amount of resultant eddy currents in the MRI device.